1. Field
This disclosure relates generally to mole or gas delivery devices, and more particularly to a method of and system for fast pulse gas delivery (PGD). As used herein the term “gas(es)” includes the term “vapor(s)” should the two terms be considered different.
2. Overview
The manufacture or fabrication of semiconductor devices often requires the careful synchronization and precisely measured delivery of as many as a dozen gases to a process tool such as a vacuum processing chamber. For purposes herein, the term “process tool” is intended to include both tools and process chambers. Various recipes are used in the manufacturing process, involving many discrete processing steps, where a semiconductor device is typically cleaned, polished, oxidized, masked, etched, doped, metalized, etc. The steps used, their particular sequence, and the materials involved all contribute to the making of particular devices.
As more device sizes have shrunk below 90 nm, one technique known as atomic layer deposition, or ALD, continues to be required for a variety of applications, such as the deposition of barriers for copper interconnects, the creation of tungsten nucleation layers, and the production of highly conducting dielectrics. In the ALD process, two or more precursor gases are delivered in pulses and flow over a wafer surface in a process tool maintained under vacuum. The two or more precursor gases flow in an alternating or sequential manner so that the gases can react with the sites or functional groups on the wafer surface. When all of the available sites are saturated from one of the precursor gases (e.g., gas A), the reaction stops and a purge gas is used to purge the excess precursor molecules from the process tool. The process is repeated, as the next precursor gas (e.g., gas B) flows over the wafer surface. For a process involving two precursor gases, a cycle can be defined as one pulse of precursor A, purge, one pulse of precursor B, and purge. A cycle can include the pulses of additional precursor gases, as well as repeats of a precursor gas, with the use of a purge gas in between successive pulses of two precursor gases. This sequence is repeated until the final thickness is reached. These sequential, self-limiting surface reactions result in one monolayer of deposited film per cycle.
The delivery of pulses of precursor gases introduced into the process tool can be controlled using on/off-type valves which are simply opened for a predetermined period of time to deliver a desired amount (mass) of precursor gas with each pulse into the processing chamber. Alternatively, a mass flow controller, which is a self-contained device comprising a transducer, control valve, and control and signal-processing electronics, is used to deliver an amount of gas (mass) at predetermined and repeatable flow rates, in short time intervals. In both cases, the amount of material (mass) flowing into the process chamber is not precisely measured and controlled.
Systems known as pulse gas delivery (PGD) devices have been developed that can deliver measured pulsed mass flow of precursor gases into semiconductor process tools. Such devices are designed to provide repeatable and precise quantities (mass) of gases for use in semiconductor manufacturing processes, such as atomic layer deposition (ALD) processes.
Single channel PGD devices each include a delivery reservoir or chamber containing the gas to be delivered during the ALD process upstream to the process tool. Gas is introduced into the delivery chamber through an inlet valve during a charging phase (when the corresponding inlet and outlet valves are respectively opened and closed), while gas is delivered from the delivery chamber through an outlet valve during a delivery phase. A pressure sensor and a temperature sensor is used to measure the pressure and temperature of the gas in the delivery chamber, and a dedicated controller is used to sense the pressure and temperature information and control the opening and closing of the inlet and output valves. Since the volume of the delivery chamber is fixed and known, the amount of gas, measured moles, delivered with each pulse is a function of the gas type, the temperature the gas in the chamber, and the pressure drop of the gas during the duration of the pulse.
Multiple channel PGD devices include multiple delivery chambers, each containing a precursor or purge gas used in a gas delivery process. Each precursor and purge gas used in a process can then be introduced through a different channel. This allows the device to operate in the charging phase for one gas provided in one channel, while delivering pulses of a gas provided in another channel. The flow of the pulse of gas from each delivery chamber is controlled with a corresponding on/off-type outlet valve between the delivery chamber of the PGD and the process tool receiving the gas. The amount of time the valve is required to be open to deliver a pulse of gas of a given mass is a further function of the starting pressures of the gas in the corresponding delivery chamber and the downstream pressure of the processing tool. For example, for a given amount of gas that needs to be delivered, the starting pressure in the delivery chamber at a higher starting pressure requires a shorter time for the valve to be open than at a lower starting pressure since the mass flow occurs more quickly at the higher starting pressure. The charge period and the delivery period of PGDs are tightly controlled for fast pulse gas delivery applications in order to insure accurate delivery of prescribed amounts of gas(es). As a result, the upstream pressure of the PGDs as well as the charged pressure in the PGDs are tightly controlled in order to meet the repeatability and the accuracy requirement of the ALD process. By using multiple channels, and staggering the charging and delivery phases of the channels, the sequential delivery of pulses of different gases can be faster than achieved by a single channel device since it is possible to charge a delivery chamber of one channel, while delivering a predetermined amount of gas from the delivery chamber of another channel.
Current multichannel PGD devices include a separate dedicated channel controller for operating each channel. Each channel controller receives all of its commands from the tool/host controller used to control the process in the tool. In this way each channel is controlled by the tool/host controller so that the entire process can be coordinated and controlled by that central controller. Thus, during a process run, the tool/host controller continually sends instruction commands to each channel controller to insure the timely and coordinated delivery of the individual pulses of gas from the multiple channels.
More recently, certain processes have recently been developed that require high speed pulsed or time-multiplexed processing. For example, the semiconductor industry is developing advanced, 3-D integrated circuits thru-silicon vias (TSVs) to provide interconnect capability for die-to-die and wafer-to-wafer stacking Manufacturers are currently considering a wide variety of 3-D integration schemes that present an equally broad range of TSV etch requirements. Plasma etch technology such as the Bosch process, which has been used extensively for deep silicon etching in memory devices and MEMS production, is well suited for TSV creation. The Bosch process, also known as a high speed pulsed or time-multiplexed etching, alternates repeatedly between two modes to achieve nearly vertical structures using SF6 and the deposition of a chemically inert passivation layer using C4F8. Targets for TSV required for commercial success are: adequate functionality, low cost and proven reliability.
The high speed processes require fast response times between successive pulses in order to better control the processes. While multichannel PGD devices have made the processes possible, in general the faster the device can transition between the alternating etch- and passivation steps the better the control of the process. Timing is very important for controlling the etching and passivation steps, particularly the time it takes to introduce the passivation gas following a etching step so that the etching step is stopped at a precise time. The faster the steps can be performed the better.
Accordingly, it is desirable to design a multichannel PGD device that can carry out high speed processes faster, without sacrificing the advantages of a multichannel PDG device.